Where devices used for observation etc are employed on an aircraft, then it is desirable to have a good field of view. For observation cameras, this good field of view has been achieved by mounting the camera on an enclosed gimbal or turret on the underside of the aircraft, so that the camera is protruding from the underside of the aircraft and can swivel about one or more axes. This mounting provides a good field of view in all directions—forwardly in the usual direction of travel of the aircraft, rearwardly in a direction facing away from the usual direction of travel, and at intermediate positions. Such cameras are already available for helicopter use, for example under the name Wescam MX-20 and so development costs are saved if these “off-the-shelf” devices can be employed.
One drawback of having a protruding camera is that it increases aerodynamic drag significantly when the aircraft is travelling at speed. This is not too problematic for relatively slow moving aircraft such as helicopters or lighter than air aircraft, but where increased air speed is required then the drag becomes a significant problem. It is not practical to retract the camera into the aircraft fuselage if it is in use.
Unmanned air vehicles (UAVs), are generally used for observation. So, cameras fitted to such air vehicles are in use for the majority of their flight, and so retracting the camera is not practicable, and in any event generally precluded for reasons of space and weight. Thus the camera needs to be exposed for use, in a drag-inducing position. Similar considerations apply to equipment designed to transmit a beam of radiation as well as or instead of receiving, and the term “field of view” should be interpreted correspondingly.